Xenon (Xe) is a noble-gas element that is extremely stable due to its closed-shell electronic structure. Indeed, this lack of reactivity is why this group of elements (He, Ne, Ar, Kr, Xe) are called noble or inert. The first Xe compounds (e.g., XeF2) were identified less than 50 years ago. Although the chemistry of this element is limited, its physical properties (e.g., ionization potential, atomic mass, electronic structure) have resulted in an increasing number of applications requiring xenon.
Demand for xenon is growing based on emerging applications in the manufacturing and healthcare industries.
Xenon is a byproduct and a slowdown of industrial production can lead to xenon shortages even as its usage increases.
Xenon is also a byproduct from etching processes that employ XeF2. This molecule decomposes on the surfaces of materials such as silicon to release atomic fluorine. The xenon then departs from the surface back into the gas phase.
Adding xenon to fluorocarbon (e.g., C4F6) plasmas used to etch silicon oxide during semiconductor manufacturing improves the anisotropy of the etch profile. Xenon additions also improve etch selectivity; i.e., the etch rate of silicon oxide is much higher than photoresist used to pattern the silicon oxide film.
Xenon also finds increasing use in the healthcare industry as an anesthetic gas, as described in U.S. Pat. No. 6,236,041, and in medical imaging as described in U.S. Pat. No. 6,408,849. Other applications include Ion Propulsion Engines (Aerospace), Flat Panel Displays (Plasma), and High Intensity Discharge (HID) Lighting.
However, a potential barrier to increased xenon utilization is the relatively high cost of xenon. Xenon is a trace component of the atmosphere (87 ppb) obtained by air separation; i.e., 11 million L of air is needed to obtain 1 L of Xenon. Consequently, Xenon is thus a high value material. Additionally, Xenon pricing is quite variable since its supply is controlled by air separation units (ASUs) supporting underlying industries, such as steel.
In U.S. Pat. No. 7,285,154,which is incorporated herein by reference in its entirety, a method has been developed for recovering xenon from gas mixtures, such as manufacturing effluent gases, using an adsorption-based process. However, the process only recovers Xenon from a nitrogen-rich, xenon-containing feed gas that is dilute in xenon (0.5% to 5.0% xenon), the final concentration is about 15 times its initial concentration.
Despite the foregoing developments, it is still desired to provide additional and improved means to recover Xenon from gaseous mixtures. And, there is a need for recovering unutilized Xe from Xe-based process with a high concentration of Xe.